Unit 5- Topic 20
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Test and notes on alkenes
Functional group= C=C Test: add bromine water (Br2 (aq)) Observation: bromine decolourises immediately
Reaction bromine water with aldehydes and ketones
Bromine water is decolourised very slowly
Test and observation of halogenoalkanes (R-X) where X= Cl, Br or I
Test: warm with NaOH (aq) and then add dilute HNO3, followed by a few drop of AgNO3(aq) Observations: R-Cl= produces white ppt. Ppt is insoluble in dilute NH3 R-Br= produces cream ppt. Ppt is insoluble in dilute NH3 but soluble in concentrated NH3 R-I= produces yellow ppt. Ppt is insoluble both dilute NH3 and concentrated
Test and observation of hydroxy group
Functional group= -OH Test= add solid PCl5 Observation= misty fumes Both alcohols and Carboxylic acids produce misty fumes, but not phenols
Test and observations of primary alcohols
Functional group= (RCH2OH) Test= add acidified K2Cr2O7 solution and warm Observation= solution turns from orange to green The organic product of this reaction produces a silver mirror with Tollen’s reagent
Test and observation of secondary alcohols
Functional group= (R2CHOH) Test=add acidified K2Cr2O7 solution and warm Observation= solution turns from orange to green Notes= the organic product does not produce a silver mirror with Tollen’s reagent
Test and observation for carbonyls
Functional group: (C=O) Test: add 2,4-DNPH Observation: orange precipitate Notes:both aldehydes and ketones produce and orange ppt
Test and observation for aldehydes
Functional group=(RCHO) Test= add Tollen’s reagent and warm Observation= silver mirror forms Notes= Fehling’s solution can also be used. Ketones do not react with Tollen’s or Fehling
Test and observation for CH3- C=O- R or CH3- CHOH - R
Test= add alkaline solution of iodine and warm Observation= yellow ppt forms Notes= known as triiodomethane or iodoform test
Test and observation for carboxylic acids
Functional group= R-COOH Test= add NaHCO3 or Na2CO3 and warm if necessary Observation= bubbles of gas
Test and observation for phenol
Functional group= benzene- OH Test= addd bromine water Observations= bromine immeaditely decolourises and a white precipitate forms Notes= phenylamine produces same results. Phenol is soluble in NaOH but insoluble in dilute HCl. Phenylamine is insoluble in NaOH but soluble in dilute HCl
State the four ways of extending an existing carbon chain by one or more atoms
1- Reacting a halogenoalkane with a cyanide ion forms a nitrile with one more carbon atom that the halogenoalkane 2- the addition of hydrogen cyanide to a carbonyl compound 3- the alkylation of benzene, which introduces an alkyl group into a benzene ring 4-the use of Grignard reagents
What are the Grignard reagents
They are organometallic compounds containing magnesium. They are made by heating under reflux the chosen halogenoalkane with magnesium in a solvent of dry ether. Bromoalkanes are the preferred halogenoalkane. Grignard reagents contain magnesium covalently bonded to both the alkyl group and the halogen. General equation: R - Br + Mg —> R - Mg - Br Grignard reagents react with water so they are made nd used in a solvent of dry ether
Grignard reactions
RMgBr + CO2 -> RCOOH, grignard + carbon dioxide -> carboxylic acid RMgBr + CH2O -> RCH2OH, grignard + methanal -> primary alcohol RMgBr + R’CHO -> RR’CHOH, grignard + aldehyde -> secondary alcohol RMgBr + R’COR’’ -> RR’R’’COH, grignard + ketone -> tertiary alcohol After the reaction is complete, dilute acid is added to obtain the desired organic product
How can functional groups influence the behaviour of compounds that contain them
-can affect if the compounds is going to act as nucleophilic or electrophile -if they are susceptible to addition reactions or substitution -if they are easily oxidised or easily reduced
How to approach a ‘plan to a reaction scheme with intermediate’ questions
-check is carbon chain length has increased or decreased -by looking at final product and working back to the starting compound -by looking at the starting compound and thinking of types of reactions it can undergo
Reaction: alkene to halogenoalkane
Equation: CH2=CH2 + HX -> CH3CH2X Reagent: hydrogen halide Conditions: mix the gases at room temperature
Reaction: halogenoalkane to alcohol
Equation: RX + NaOH —> ROH + NaX Reagent: aqueous sodium hydroxide Conditions: heat under reflux
Halogenoalkane to nitrile
Equation: RX + KCN —> RCN + KX Reagent: alcoholic potassium cyanide Conditions: heat under reflux
Reaction: halogenoalkane to amine
Equation: RX + 2NH3 —> RNH2 + NH4X Reagent: aqueous ammonia Conditions: heat under reflux
Reaction: alcohol to chloroalkane
Equation: ROH + PCl5 —> RCl + HCl + POCl3 Reagent: phosphorus (V) chloride Conditions: room temperature
Reaction: alcohol to bromoalkane
Equation: ROH + HBr —> RBr + H2O Reagent: 50% concentrated sulfuric acid and potassium bromide Conditions: warm
Reaction: alcohol to iodoalkane
Equation: 3ROH + PI3 —> 3RI + H3PO3 Reagent: red phosphorus and iodine Conditions: heat under reflux
Reaction: primary alcohol to aldehyde
Equation: RCH2OH + [O] —> RCHO + H2O Reagent: potassium dichromate (Vl) and dilute sulfuric acid Conditions: add the reagent to hot alcohol and allow the aldehyde to distil off as it is formed